Secondary batteries including a non-aqueous electrolyte (i.e., non-aqueous electrolyte secondary batteries) such as lithium ion secondary batteries are generally charged by a method of constant-current and constant-voltage charging. In constant-current and constant-voltage charging, charging is performed at a constant current until the battery voltage reaches a predetermined voltage. After the battery voltage has reached the predetermined voltage, the charge current is reduced so that the battery voltage is maintained at the predetermined voltage. When the charge current is reduced to a predetermined value, charging is terminated.
With regard to constant-current and constant-voltage charging, Patent Literature 1 discloses the following technique.
In constant-current and constant-voltage charging, if the charge cutoff voltage of a battery is set high, the decomposition of the electrolyte on the positive electrode and the destruction of crystals of the positive electrode active material are accelerated, and the cycle characteristics deteriorate. In order to avoid this, charging is performed at a large current at the beginning of charging. The charge current is reduced immediately after the battery voltage has reached a charge cutoff voltage. By reducing the charge current, the battery voltage is reduced. When the battery voltage reaches the charge cut-off voltage again, the charge current is further reduced. These procedures are repeated. In such a manner, charging is performed at a large current at the beginning of charging, and then the charge current is reduced stepwise.
Patent Literature 2 proposes the following technique.
In constant-current charging, rapid charging is performed at a comparatively large current of 1.2 to 4.0 It while the state of charge is low. After the state of charge (SOC) is increased, charging is performed at a current of 1.2 It or less. By doing this, the charging time can be shortened, and the deterioration in cycle characteristics can be suppressed.